An inertial unit is a precision navigation device that comprises in particular gyroscopes in order to be able to provide navigation information for carriers such as ships, aircraft, missiles or all types of space vehicle.
The level of precision required for the measurements provided by the gyroscopes used in these inertial units depends on the type of inertial unit considered. Certain units, such as strapdown inertial navigation systems, i.e. navigation units in which the gyroscopes are strapped to the structure of the carrier, require a level of precision of the measurement provided by the gyroscopes greater than that which is required in other types of unit.
In order for a strapdown inertial navigation system to be capable of providing precise and reliable measurements, laser gyroscopes are conventionally used, as they have a high quality scale factor. However, this type of gyroscope is expensive.
It therefore appears advantageous to be able to replace them with less expensive gyroscopes, such as HRG-type vibratory gyroscopes (for ‘Hemispherical Resonator Gyroscope’).
An axisymmetric vibratory gyroscope (these Type I gyroscopes correspond to the axisymmetric gyroscopes of the Coriolis Vibratory Gyroscope (CVG) type, such as for example of the Hemispherical Resonator Gyroscope (HRG) type, as described in the document ‘Type I and Type II micromachined vibratory gyroscopes’ by Andrei M. Shkel, pages 586-593, IEEE/ION (Institute of Electrical and Electronics Engineers/Institute Of Navigation' PLANS 2006, San Diego, Calif., USA)) can be used either in whole angle mode in which its vibration position is free, or in a rate mode in which its vibration position is fixed and maintained by application of an electrical command, the measurement values provided being deduced from the electrical command applied, as well as from a value of the scale factor associated with the vibratory gyroscope(s) used.
By the term ‘scale factor’ is meant here the factor between the rotation of the vibration wave of the vibratory gyroscope and the rotation of the support of the vibratory gyroscope, i.e. the ratio relating to the drive of the rotation of the vibration wave of the gyroscope by the mechanical rotation of the support of the gyroscope. In theory, this scale factor for a given vibratory gyroscope is a constant value that can correspond to a nominal value associated with the gyroscope considered.
Moreover, such a gyroscope used in whole angle mode has a bias error the value of which depends on the vibration position with respect to the case.
The measurement values provided by such a vibratory gyroscope can therefore be impaired by certain errors that relate to the changes in the bias, since the bias value depends on the vibration position and therefore on the changes in the attitude of the carrier.
It should moreover be noted that a use of such a vibratory gyroscope in whole angle mode makes it possible to provide measurement values with errors relating to the scale factor that are advantageously small. On the other hand, in this mode, the measurement values, which are angle values, provided by the vibratory gyroscope, are impaired by a noise known as ‘angular noise’.
In order to avoid this variable bias as well as this angular noise, the vibratory gyroscope can be used in a rate mode.
However, such an operating mode can significantly degrade the level of quality of the measurement as in this case errors relating to the scale factor appear. Therefore, in such an operating mode, these vibratory gyroscopes cannot satisfy the quality criteria required in particular for a strapdown inertial system.